Immobiliser Diagnostics & Repair Devon, Dorset & Somerset — On-Site, Same Day
If your engine cranks normally but won't start, or you're seeing a red padlock or car outline warning light on your dashboard, your vehicle immobiliser may not be recognising your key. This isn't always a hardware fault — in many cases it's a re-pairing or re-synchronisation issue we resolve on-site using our AUTEL diagnostic equipment.
Is My Immobiliser the Problem? Recognising the Symptoms
The vehicle immobiliser is a mandatory anti-theft system in all UK cars manufactured from 1998 onwards. Its job is to prevent the engine from starting without the correct key signal. When it works correctly, you never notice it. When it develops a fault — or loses its pairing with your key — the engine cranks perfectly but simply won't fire. There are no mechanical noises, no fuel smell, just an engine that turns over and over but never starts.
The most important thing to understand: immobiliser issues are usually electronic, not mechanical. The car is not broken. The ECU is doing exactly its job — blocking the engine because it doesn't recognise the key's transponder signal. In many cases, the fix is a re-synchronisation or key re-pairing procedure, not a parts replacement.
The four most common immobiliser fault symptoms
Engine Cranks But Won't Fire
The starter motor works perfectly — the engine turns over — but combustion never happens. Fuel delivery or ignition is being blocked. This is the classic immobiliser symptom.
Red Padlock Warning Light
A red padlock, key outline or car-with-key symbol on your dashboard indicates the ECU has detected an immobiliser fault or cannot authenticate the key signal.
Key No Longer Recognised
A key that started the car yesterday suddenly doesn't work today. Often happens after a flat battery, battery replacement, or a power interruption during a service.
All Keys Failed Simultaneously
All registered keys stopped working at the same time — typically points to an ECU fault rather than a key issue, since it's unlikely all keys failed individually at once.
📍 Real Callout — Exeter, Devon
"A Toyota Prius owner in Exeter called us after the 12v auxiliary battery went completely flat. When jump-started, the car turned over fine but the engine wouldn't fire — and the immobiliser warning light was lit. Hybrid vehicles have a specific re-synchronisation requirement after a 12v power loss that differs from standard petrol cars. Our AUTEL diagnostic software covers the Prius-specific procedure and the car was back in use within 40 minutes of our arrival."
📍 Real Callout — Sherborne, Somerset
"A BMW 3 Series owner in Sherborne had his ECU replaced by an independent garage after a fault. Afterwards, none of his three keys would start the car. The replacement ECU had no key data registered to it — the garage hadn't performed the key re-pairing procedure after fitting the new unit. We attended his home, connected our AUTEL BimTool software, and re-paired all three keys to the replacement ECU. Two hours from first call to driving away."
📍 Real Callout — Plymouth, Devon
"Ford PATS (Passive Anti-Theft System) faults are the most common immobiliser callout we see around Plymouth and Exeter. A Ford Focus owner called us after her car developed an intermittent no-start issue — it would start fine some days and then refuse completely on others. Our AUTEL read a stored PATS fault code. The transponder in the key head had developed an intermittent connection — common on older flip key housings. A new key head programmed to the existing ECU resolved the issue completely."
⏱ When does an immobiliser fault occur?
Common Immobiliser Fault Types We Diagnose & Resolve
AUTEL diagnostic equipment reads fault codes and identifies the exact cause before any work begins
Key–ECU Re-Synchronisation
The most frequent immobiliser fault we encounter — the key and ECU have lost their pairing. This often follows a flat battery, battery replacement, power interruption or vehicle service where the battery was disconnected. The key itself is fine, the ECU is fine — they've simply lost communication. A re-synchronisation procedure via OBD re-establishes the pairing without any parts replacement.
ECU Replacement — Key Re-Pairing
When a vehicle ECU is replaced — whether due to a fault, water damage or accident — all previously registered keys lose their pairing to the new unit. The new ECU has no memory of any keys. This requires re-pairing every key to the replacement ECU via OBD. It's a job that garages without specialist equipment cannot perform, and the reason many vehicles end up stranded after what should be straightforward ECU work.
Failed or Intermittent Transponder Chip
The transponder chip inside the key head can fail due to physical damage, water ingress, extreme heat or simply age. An intermittent chip failure produces an unreliable no-start fault — the car starts fine sometimes and then refuses completely. Our AUTEL equipment reads the specific fault codes that identify a transponder chip issue. Resolution is a new key head programmed to the existing ECU.
ECU Immobiliser Module Fault
In some cases the ECU's immobiliser module itself has developed a fault — this is less common than key or pairing issues but does occur, particularly on higher-mileage vehicles. Our AUTEL diagnostics can distinguish between a module fault and a pairing issue, which matters because the resolution approach and cost are very different. We'll give you a clear explanation of what we find before any work begins.
Power Loss — Immobiliser Memory Reset
Some vehicle models are particularly sensitive to power loss — Ford, Vauxhall and Peugeot models are the most commonly affected. A completely flat battery or jump start can cause the immobiliser module to lose one or more key registrations. The key worked perfectly before the battery issue, then one or all keys stopped being recognised. This is a re-pairing or re-registration procedure — not a fault with the key or ECU hardware.
Make-Specific Immobiliser Systems
Different manufacturers use different immobiliser architectures — Ford PATS, VW/Audi Immo3/Immo4, BMW EWS/CAS/FEM, Vauxhall BCM-based systems. Each requires specific diagnostic software and procedures. Our AUTEL equipment carries manufacturer-specific software for the major makes on UK roads, allowing us to read fault codes, perform re-synchronisation and re-pair keys across virtually all common systems.
Our Diagnostic Process
We identify the exact fault before quoting — no guesswork, no unnecessary parts
You Call
Describe the symptoms — engine cranks but won't start, warning light on, key not recognised. We ask a few quick diagnostic questions and give you an ETA.
We Arrive
AUTEL MaxiSys connected to vehicle OBD port. Full immobiliser fault code scan — we identify exactly what the ECU is reporting before any work begins.
Clear Diagnosis
We tell you exactly what the fault is, what's needed to fix it, and the cost — before we start. No hidden charges. Most common faults are resolved in the same visit.
Resolved & Tested
Re-synchronisation or key re-pairing completed. Engine start tested. Every key verified working before we leave. Job done — no towing, no waiting.
Make-Specific Immobiliser Systems We Work With
Different manufacturers use different architectures — our AUTEL software covers them all
Ford
PATS — Passive Anti-Theft System
Ford's PATS system is the most common immobiliser fault we attend across Devon and Somerset. The system uses a transponder ring around the ignition barrel to read the key chip. PATS faults range from single-key loss events to full system resets. Our AUTEL reads all PATS fault codes and performs key re-registration and re-synchronisation for the full Ford range.
Ford locksmith service →VW / Audi / Seat / Skoda
Immo3 / Immo4 — BSI-integrated
The VW Group uses Immo3 on older vehicles and Immo4 on post-2009 models. Immo4 is significantly more complex, requiring specific AUTEL software channels for key re-pairing. All four VW Group brands share the same underlying architecture — making them one of the most common ECU re-pairing callouts we attend across Plymouth and Exeter.
Call 07830249408 to confirm your model →BMW
EWS2 / EWS3 / CAS / FEM systems
BMW has used several immobiliser generations. EWS systems on pre-2005 vehicles, CAS (Car Access System) on 2005–2018 models, and FEM/BDC on post-2015 Series. Our AUTEL BimTool Pro software covers the full BMW range for key re-pairing after ECU work, battery-related re-synchronisation and lost-key new key programming.
Call 07830249408 for BMW immobiliser help →Vauxhall / Opel
BCM-integrated immobiliser
Vauxhall uses a BCM (Body Control Module) integrated immobiliser system. Key re-pairing requires Tech2-compatible software — which our AUTEL carries. Vauxhall Astra and Corsa immobiliser faults after battery work are particularly common in our coverage area, and are resolved on-site in most cases.
Vauxhall locksmith service →Peugeot / Citroën
BSI (Built-in Systems Interface)
French vehicles use the BSI module to manage both immobiliser functions and body electronics. BSI faults are common after battery work and can prevent any keys from being recognised. Our AUTEL covers BSI re-initialisation, key re-pairing and fault code clearing for the full Peugeot and Citroën range.
Peugeot locksmith service →Toyota / Lexus (incl. Hybrid)
Standard & Smart Entry systems
Toyota hybrids — particularly the Prius, which is common around Exeter — have a specific 12v auxiliary battery re-synchronisation requirement after power loss. This differs from standard petrol vehicle procedures and is a common source of confusion. Our AUTEL carries the Toyota/Hybrid-specific software required for correct re-synchronisation.
Honda & Japanese makes →Recent Immobiliser Diagnostic Callouts
Recent jobs resolved across Devon, Dorset & Somerset
Toyota Prius — 12v battery loss, immobiliser re-synchronisation
BMW 3 Series — ECU replaced by garage, all keys re-paired
Ford Focus — PATS fault after battery work, key re-registered
Vauxhall Astra — BCM re-sync after flat battery, both keys verified
Peugeot 308 — BSI fault cleared, key re-paired to module
VW Transporter T6 — Immo4 fault, key re-registration completed
Updated regularly · Call 07830249408 with your symptoms for a rapid ETA
Dean — DB Car Keys
Mobile Auto Locksmith & Immobiliser Specialist · Honiton, Devon
What Our Customers Say
Verified reviews from drivers across Devon, Dorset & Somerset
Vehicle Makes We Cover for Immobiliser Diagnostics
AUTEL diagnostic software for all major manufacturer immobiliser systems
Call 07830249408 with your registration — we confirm what we can do before travelling.
Immobiliser Service Areas
On-site immobiliser diagnostics at your location — all three counties
EX, PL, TQ postcodes — Exeter, Plymouth and all Devon towns
DT postcodes — Bridport, Weymouth, Dorchester and coastal towns
TA & BA postcodes — Taunton, Yeovil, Sherborne and rural Somerset
Call 07830249408 to confirm · EX · PL · TQ · DT · TA · BA postcodes
Immobiliser Diagnostics — FAQs
Common questions about immobiliser faults and our diagnostic service
Related: Car Key Programming · Transponder Keys · Key Fob Repairs · Lost Car Keys